1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser used as a source for optical information processing, optical communications, optical recording and image forming. The present invention also relates to a method and apparatus for fabricating such a surface emitting semiconductor laser. More particularly, the present invention relates to a technique of accurately defining an aperture surrounded by a selectively oxidized portion of a current confinement region.
2. Description of the Related Art
Recently, there has been an increased demand for a surface emitting semiconductor laser capable of easily realizing an array of sources in the technical fields of optical communications and optical interconnections. Such a laser is also called vertical-cavity surface-emitting laser diode (VCSEL).
The surface emitting semiconductor laser is categorized into a proton injection type with a gain waveguide structure, and a selective oxidization type with a refractive ratio waveguide structure. Nowadays, the latter is getting the mainstream. Generally, the selective oxidization type semiconductor laser has a laser portion of a mesa structure. A current narrowing or confining region formed by selectively oxidizing part of an AlAs layer or AlGaAs layer is formed in the vicinity of the active region of the mesa. The current confinement layer increases the resistivity and reduces the refractive index. This results in an optical waveguide path.
The degree of dimensional accuracy of the non-oxidized region surrounded by an aperture of the current confinement layer and defined by the selectively oxidized region is a very important factor that determines the device performance. The threshold current of laser and the transverse oscillation mode greatly depend on the diameter of the aperture.
Generally, the aperture of the current confinement region is formed by a wet oxidization method. This method employs a carrier gas of nitrogen, and transports a pure water vapor heated up to approximately 100° C. to a chamber. The AlAs or AlGaAs layer having the side surface exposed is oxidized therefrom.
However, it is very difficult to reproducibly control the distance of oxidation that advances from the side surface of the mesa on the process basis and to form the aperture of the current confinement layer as designed. This is because the mesa may have precision error caused by etching, and the oxidization rate depends on the temperature of the water vapor, the amount of gas transported, and the thickness of a naturally oxidized film on the side surface of the AlAs or AlGaAs layer.
A proposal to solve the above problems is described in Japanese Unexamined Patent Publication No. 2001-93897. The proposal describes the use of a sample for monitoring and tracks the degree of advance of oxidization reaction on the wafer or substrate, with which degree the oxidization reaction of the current confinement layer is controlled. The monitor-use sample and the current confinement layer on the wafer or substrate are put in an oxidizing chamber and are subject to simultaneous oxidizing. The oxidization reaction on the current confinement region is controlled by monitoring a reflected light that is varied due to change of the oxidized region of the monitor-use sample.
However, the proposal heats the temperature in the oxidizing chamber to 400° C. for oxidization for forming the current confinement layer. Thus, the AlAs or AlGaAs layer is oxidized at a comparatively high rate of oxidization. Even when the oxidization reaction is terminated by monitoring the reflected light from the monitor-use sample, oxidization of the AlAs or AlGaAs layer from the side surface of the mesa may advance to some extent, so that the aperture of the current confinement layer (diameter of the aperture) cannot fall within the design range. Further, the production yield will be degraded if the aperture defined by the current confinement layer is not reproduced accurately on the process basis. This drives up the cost of producing the surface emitting semiconductor laser devices.